libgo patch committed: Synchronize empty struct field handling

[Ian Lance Taylor <iant@golang.org>]

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This libgo patch by Funan Zeng synchronizes the handling of empty
struct fields between the Go frontend and the libgo FFI code.  In the
compiler the logic for allocating one byte for the last field of a
struct is:
1. the last field has zero size
2. the struct itself does not have zero size
3. the last field is not blank
This patch adds the last two conditions to runtime.structToFFI.  This
is for https://go.dev/issue/55146.  Bootstrapped and ran Go testsuite
on x86_64-pc-linux-gnu.  Committed to mainline.

Ian

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085bacba3502ff77c70a7660c19a68f50e9b7877
diff --git a/gcc/go/gofrontend/MERGE b/gcc/go/gofrontend/MERGE
index f7a7985287d..73aa712dbdf 100644
--- a/gcc/go/gofrontend/MERGE
+++ b/gcc/go/gofrontend/MERGE
@@ -1,4 +1,4 @@
-42efec8c126cf3787bc7c89d9c7f224eff7c5a21
+0140cca9bc0fad1108c7ed369376ac71cc4bfecf
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
diff --git a/libgo/go/runtime/ffi.go b/libgo/go/runtime/ffi.go
index cd8479ef551..86ce5b85d04 100644
--- a/libgo/go/runtime/ffi.go
+++ b/libgo/go/runtime/ffi.go
@@ -4,6 +4,7 @@
 
 // Only build this file if libffi is supported.
 
+//go:build libffi
 // +build libffi
 
 package runtime
@@ -221,9 +222,6 @@ func stringToFFI() *__ffi_type {
 // structToFFI returns an ffi_type for a Go struct type.
 func structToFFI(typ *structtype) *__ffi_type {
 	c := len(typ.fields)
-	if c == 0 {
-		return emptyStructToFFI()
-	}
 	if typ.typ.kind&kindDirectIface != 0 {
 		return ffi_type_pointer()
 	}
@@ -231,6 +229,7 @@ func structToFFI(typ *structtype) *__ffi_type {
 	fields := make([]*__ffi_type, 0, c+1)
 	checkPad := false
 	lastzero := false
+	sawnonzero := false
 	for i, v := range typ.fields {
 		// Skip zero-sized fields; they confuse libffi,
 		// and there is no value to pass in any case.
@@ -239,10 +238,13 @@ func structToFFI(typ *structtype) *__ffi_type {
 		// next field.
 		if v.typ.size == 0 {
 			checkPad = true
-			lastzero = true
+			if v.name == nil || *v.name != "_" {
+				lastzero = true
+			}
 			continue
 		}
 		lastzero = false
+		sawnonzero = true
 
 		if checkPad {
 			off := uintptr(0)
@@ -263,6 +265,10 @@ func structToFFI(typ *structtype) *__ffi_type {
 		fields = append(fields, typeToFFI(v.typ))
 	}
 
+	if !sawnonzero {
+		return emptyStructToFFI()
+	}
+
 	if lastzero {
 		// The compiler adds one byte padding to non-empty struct ending
 		// with a zero-sized field (types.cc:get_backend_struct_fields).